Flow retarder for bearing assembly of downhole drilling motor

ABSTRACT

A flow retarder is provided for use in the bearing assembly of a downhole drilling motor. The bearing assembly comprises an inner tubular mandrel and an outer tubular housing. The flow retarder comprises: an inner carbide sleeve connected to the rotating mandrel by a resilient elastomer layer bonded to a driver ring which is locked on the rotating mandrel; an outer carbide sleeve affixed to the stationary tubular housing; the sleeves forming a narrow annular clearance or metering gap between them; a secondary port extending through the mandrel side wall directly above the carbide sleeves; and a housing port extending through the housing side wall below the flow retarder sleeves. Drilling fluid leaks down from the adapter/adjustable housing clearance and passes through the metering gap to lubricate the carbide sleeves. Part of this fluid returns through the secondary port to the main bore of the mandrel. By providing the elastomer layer, the transmission of radial loads from the inner sleeve to the outer sleeve is reduced.

FIELD OF THE INVENTION

[0001] The present invention relates to a fluid retarder or restrictor used in the bearing assembly of a downhole drilling motor.

BACKGROUND OF THE INVENTION

[0002] A downhole drilling motor and its bearing assembly are known tools used in well drilling. The present invention is concerned with improving the bearing assembly.

[0003] A bearing assembly typically includes:

[0004] an inner rotating tubular mandrel connected between the power unit of the downhole drilling motor and the bit;

[0005] an outer stationary tubular housing co-extensive with the mandrel and connected with the drill string;

[0006] bearings positioned in a bearing chamber formed between the mandrel and housing;

[0007] spaced stationary and floating piston seals closing the ends of the bearing chamber, which is filled with lubricating oil;

[0008] the floating piston seal being positioned at the lower end of an annular pressure balancing chamber, formed between the mandrel and housing.

[0009] The bearing assembly is connected at its upper end to the motor power section by an inner tubular adapter and an outer tubular adjustment housing. Drilling fluid moving down through the clearance between the adapter and the adjustment housing enters the bore of the mandrel through a primary mandrel port. This clearance can be referred to as the main clearance.

[0010] An assembly referred to as a flow retarder is positioned at the upper end of the pressure balancing chamber. A small portion of the drilling fluid moves down to the flow retarder through a narrower extension of the main clearance. The prior art flow retarder typically comprises:

[0011] a tungsten carbide inner sleeve affixed to the outside surface of the rotating mandrel;

[0012] an opposed tungsten carbide outer sleeve affixed to the inside surface of the stationary housing in opposed relationship to the inner sleeve;

[0013] a radial port extending through the housing side wall below the carbide sleeves, for connecting the pressure balancing chamber with the well borehole;

[0014] the sleeves defining a narrow annular metering gap between them; and

[0015] the upper end of the gap communicating with the aforesaid main clearance and the lower end of the gap communicating with the pressure balancing chamber.

[0016] The two sleeves provide a pressure-resistant hydrodynamic seal at the upper end of the pressure balancing chamber. The seal is provided with the objective of isolating the pressure balancing chamber from the relatively high pressure of the fluid in the main clearance, so that it experiences only the relatively low pressure of the wellbore annulus external of the bearing assembly. The gap, and the leakage of drilling fluid through it, also functions to ensure lubrication of the two sleeves (one of which is rotating and one of which is stationary).

[0017] Now, there are problems associated with these known flow retarders. More particularly:

[0018] wear of the sleeves is substantial and is exacerbated by the flow of fluid therethrough and radial loading from the rotating mandrel;

[0019] to minimize wear from radial loading, the gap is widened—this leads to a significant flow of drilling fluid through the flow retarder—this increases sleeve and housing port wear and reduces the volume of fluid reaching the bit; and

[0020] in addition, drilling fluid coarse solids from the main clearance may pile up at the upper end of the metering gap and penetrate into the gap, thus causing wear of the sleeves.

[0021] It is the objective of the present invention to provide a flow retarder in a bearing assembly which addresses these problems.

SUMMARY OF THE INVENTION

[0022] The present invention is directed to a flow retarder for use in the bearing assembly of a downhole drilling motor. The flow retarder is associated with the upper section of the pressure balancing chamber. As previously stated, the pressure balancing chamber contains an annular floating piston seal extending around the rotating inner mandrel. The flow retarder functions to restrict the flow of drilling fluid through the pressure balancing chamber, to substantially isolate the balancing chamber from the high pressure in the drilling fluid moving through the main clearance and to substantially equalize the pressure internal of the balancing chamber with the pressure of the wellbore.

[0023] The flow retarder comprises: an erosion-resistant inner sleeve connected to the rotating mandrel by a layer of resilient elastomer bonding the inner sleeve to a driver ring which in turn is locked on the rotating mandrel; an erosion-resistant outer sleeve fixed to the inside surface of the stationary tubular outer housing of the bearing assembly; the inner sleeve being positioned within the outer sleeve, the sleeves forming a narrow annular metering gap between them; the sleeves being positioned at the upper end of the balancing chamber and extending there across; a secondary mandrel port extending through the mandrel side wall adjacent the upper end of the metering gap, to provide communication between the lower end of the main clearance and the bore of the mandrel; and a housing port extending through the housing side wall to provide communication between the other end of the metering gap, the balancing chamber and the well bore.

[0024] The layer of resilient elastomer between the inner sleeve and the mandrel functions as a shock absorber for the radial loads transmitted to the inner sleeve by the rotating mandrel during drilling motor operation. As a consequence, wear on the sleeves is reduced and the annular metering gap can be made narrower, thereby reducing the loss of drilling fluid through the flow retarder.

[0025] The secondary mandrel port enables drilling fluid moving out of the main clearance and unable to penetrate through the metering gap, to circulate into the bore of the mandrel and provide a flushing function. In prior art flow retarders, particles pile up at the upper end of the carbide sleeves. These particles ultimately get ground up in the annular gap between the two sleeves, thus causing extra wear. In the present flow retarder, the pressure at the secondary mandrel port will be slightly lower than the pressure at the primary mandrel ports and the small flow of fluid into the mandrel bore will wash or flush particles away from the entrance to the metering gap.

[0026] In a preferred feature, one or more resilient belleville spring washers are positioned at the upper end of the inner sleeve and are downwardly biased to apply a downward force on the driver ring to seat the ring and sleeve on the rotating mandrel. The spring washers function as a shock absorber for the inner sleeve by allowing it to move axially a small amount during drilling motor operation, thereby reducing damage to the inner sleeve caused by vibrations in the bearing assembly.

[0027] In summary then, the inner sleeve is cushioned both radially and axially, with consequent wear reduction.

[0028] In another preferred feature, the inner sleeve has a plurality of circumferential, spaced-apart grooves extending around its outer peripheral surface. These grooves function to further restrict and reduce the flow of drilling fluid through the flow retarder.

[0029] Broadly stated, the invention is directed to a flow retarder for use in a downhole drilling motor bearing assembly used in a well borehole, said bearing assembly comprising a main clearance through which drilling fluid can move, an inner rotatable tubular mandrel having a side wall forming a longitudinal bore, an outer stationary tubular housing having a side wall, an annular pressure balancing chamber formed between the housing and mandrel side walls and an annular floating piston seal closing the bottom end of the chamber, the flow retarder comprising an erosion-resistant inner sleeve mounted on a driver ring connected to rotate with the mandrel, said inner sleeve being secured to the driver ring by a layer of resilient elastomer positioned between the sleeve and the driver ring; an erosion-resistant outer sleeve fixed to the inside surface of the housing side wall; the inner sleeve being positioned within the outer sleeve, said sleeves forming a narrow annular metering gap between them, the sleeves being positioned in the upper end of the chamber and extending there across; the housing side wall forming a housing port extending therethrough below the sleeves to connect the pressure balancing chamber and the lower end of the metering gap with the borehole.

DESCRIPTION OF THE DRAWING

[0030]FIG. 1 is a cross-sectional side view of a downhole drilling motor bearing assembly incorporating a flow retarder in accordance with the invention;

[0031]FIG. 2 is a larger scale side view of part of the bearing assembly of FIG. 1, showing the flow retarder;

[0032]FIG. 3 is a sectional side view of the inner sleeve, elastomer layer and driver ring; and

[0033]FIGS. 4, 5 and 6 are sectional side, external side and end views of the filter ring.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] The bearing assembly 1 shown in FIG. 1 comprises an inner mandrel 2 and an outer housing 3.

[0035] The mandrel 2 is connected at its upper end with an adapter 4 connectable to the power unit (not shown) of a downhole drilling motor (also not shown). The outer housing 3 is connected at its upper end with an adjustment housing 41. An annular main clearance 42 is formed between the adapter 4 and the adjustment housing 41. Drilling fluid from the motor passes down through the main clearance 42 and enters the bore 5 of the mandrel 2 through a primary mandrel port 43. A small portion of the drilling fluid can penetrate downwardly through a narrow extension 42 a of the main clearance 42, present between mandrel 2 and outer housing 3.

[0036] At its lower end the mandrel 2 is threadably connected with a bit assembly (not shown).

[0037] In use the mandrel 2 is rotatably driven and transmits drilling fluid down through its axial bore 5. The drilling fluid in the main clearance 42 and mandrel bore 5 is at higher pressure than the drilling fluid in the borehole external of the bearing assembly 1.

[0038] The bearing assembly outer housing 3 comprises a piston housing 6, a thrust housing 7 and a bearing housing 8, threadably connected together end to end.

[0039] A journal bearing assembly 9, on bottom thrust bearing 10 and off bottom thrust bearing 11 are positioned between the mandrel 2 and outer housing 3.

[0040] A stationary bottom seal assembly 12 and floating piston seal 13 seal the ends of the bearing chamber 14 containing the bearings 9, 10 and 11.

[0041] An annular pressure balancing chamber 15 is formed between the mandrel 2 and the piston housing 6. The floating piston seal 13 floats at the lower end of the pressure balancing chamber 15.

[0042] The flow retarder 16 is associated with the pressure balancing chamber 15, the mandrel 2 and the outer housing 3.

[0043] More particularly, the flow retarder 16 comprises a driver ring 17 which conforms with and seats on a tapered shoulder 18 formed by the mandrel 2. The driver ring 17 has a relatively thick base 19 and an upwardly extending thin section 20. On its inner surface, the base 19 carries an O-ring 36 to block the influx of fluid to the tapered shoulder 18. The mandrel 2 rotatably drives the driver ring 17.

[0044] An annular disc 21 is seated on the shoulder 22 formed at the junction of the driver ring base 19 and thin section 20. The disc 21 is made of aluminum and acts as a thrust cushion for the inner sleeve 23, which seats on it.

[0045] The annular inner sleeve 23 is formed of hard, erosion-resistant material such as tungsten carbide. It is bonded by a layer 24 of elastomer with the thin section 20 of the driver ring 17 and thus is drivably connected with the mandrel 2. The layer 24 is formed before assembly of the tool by injection of nitrile rubber into an annular space defined between the sleeve 23 and the thin section 20.

[0046] The outer surface 25 of the sleeve 23 is formed to provide circumferential grooves 26 at spaced intervals along its length.

[0047] An outer sleeve 27, also formed of tungsten carbide or the like, is affixed to the inside surface 28 of the piston housing 6 with a shrink fit. The outer sleeve 27 is co-extensive with and extends around the inner sleeve 23. It is positioned opposite the inner sleeve 23 by a spacer 37 which abuts an inwardly projecting shoulder 38 of the piston housing 6.

[0048] The sleeves 23, 27 are slightly spaced apart to form a metering gap 29 having a width in the order of 0.002″-0.004″.

[0049] A filter ring 30 is positioned at the upper ends of the sleeves 23, 27. The filter ring assembly 30 forms transverse slots 31 at spaced intervals at its upper end. The slots 31 are provided to allow large particles suspended in the drilling fluid to be carried away through the secondary mandrel port 32. The secondary mandrel port 32 extends radially through the side wall 33 of the mandrel 2 at a point just above the upper end of the metering gap 29.

[0050] A housing port 34 extends radially through the side wall 35 of the piston housing 6 at a point below the bottom end of the metering gap 29 and above the floating piston seal 13. The housing port 34 enables pressure communication between the lower end of the metering gap 29 and the annulus or wellbore outside the bearing assembly 1.

[0051] A ring 37 and a stack of belleville springs 38 are positioned between the upper face 39 of the filter ring 30 and the bottom face 40 of the adapter 4. As the adapter 4 is threaded onto the upper end of the mandrel 2, the face 40 presses down on the belleville springs 38, thereby resiliently biasing the driver ring 17 down to seat it firmly on the mandrel's tapered shoulder 18.

[0052] From the foregoing, it will be noted that the inner sleeve 23 is cushioned with respect to radial and axial loads.

[0053] In operation, some drilling fluid at relatively high pressure penetrates down through the narrow extension 42 a of the main clearance 42 and reaches the flow retarder 16. One part of this fluid moves into the mandrel bore 5 through the secondary mandrel port 32 and flushes coarse solids away from the top end of the metering gap 29. Another part of the fluid moves through the gap 29 to lubricate the sleeves 23, 27. The resilient elastomer layer 24 acts as a radial shock absorber for side loads originating from the rotating mandrel 2. The belleville springs 36 act to cushion the inner sleeve from axial loads. The metering gap 29 substantially isolates the pressure balancing chamber 15 from the high pressure fluid. And the housing port 34 provides communication between the pressure balancing chamber 15 and the wellbore annulus. 

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A flow retarder for use in a downhole drilling motor bearing assembly used in a well borehole, said bearing assembly comprising a main clearance through which drilling fluid can move, an inner rotatable tubular mandrel having a side wall forming a longitudinal bore, an outer stationary tubular housing having a side wall, an annular pressure balancing chamber formed between the housing and mandrel side walls and an annular floating piston seal closing the bottom end of the chamber, the flow retarder comprising: an erosion-resistant inner sleeve mounted on a driver ring connected to rotate with the mandrel, said inner sleeve being secured to the driver ring by a layer of resilient elastomer positioned between the sleeve and the driver ring; an erosion-resistant outer sleeve fixed to the inside surface of the housing side wall; the inner sleeve being positioned within the outer sleeve, said sleeves forming a narrow annular metering gap between them, the sleeves being positioned in the upper end of the chamber and extending there across; the housing side wall forming a housing port extending therethrough below the sleeves to connect the pressure balancing chamber and the lower end of the metering gap with the borehole.
 2. The flow retarder as set forth in claim 1 wherein: the mandrel side wall forms a secondary mandrel port at the upper end of the metering gap for connecting the lower end of the main clearance with the mandrel bore.
 3. The flow retarder as set forth in claim 2 comprising: the mandrel having an outwardly protruding shoulder; the driver ring being seated on the shoulder and having a frictional engagement with the mandrel so as to rotate therewith; the elastomer layer being bonded to the driver ring and the inner sleeve so that the inner sleeve rotates with the mandrel.
 4. The flow retarder as set forth in claim 3 comprising: means for resiliently pressing the driver ring against the mandrel shoulder.
 5. The flow retarder of claim 1, 2, 3 or 4 wherein: the inner sleeve has a plurality of circumferential grooves formed in its outer surface at spaced intervals along its length. 